1. Field of the Invention
This invention relates to a superconducting apparatus comprising a cryogenic chamber for storing superconducting equipment provided with superconducting magnets and/or Josephson devices etc. and, more particularly, it relates to a superconducting apparatus capable of improving the withstand voltage in peripheral areas of the leads for feeding the superconducting equipment with power from an external power source and shutting off various electromagnetic noises.
2. Related Background Art
FIG. 16 of the accompanying drawings shows a schematic sectional view of a conventional superconducting apparatus comprising a cryogenic chamber containing a superconducting magnet. Referring to FIG. 16, the apparatus comprises a superconducting coil 3 and a pair of leads 1 secured to a cryogenic chamber 2 by means of a respective fitting flanges 5. The fitting flanges 5 are typically made of metal and secured to the cryogenic chamber 2 with insulators 10 interposed therebetween. Reference numeral 4 denotes a cooling agent for cooling the superconducting coil 3. Liquid helium is typically used as the cooling agent 4 and evaporated helium gas cools down the conductors of the leads and then fed out of the cryogenic chamber by way of gas outlets 9. FIGS. 15A and 15B of the accompanying drawings respectively show in schematic cross section a longitudinal view and a plan view of a lead adapted for used with a superconducting apparatus of the type under consideration. In FIGS. 15A and 15B, the lead is housed in a hollow tube 24 covered by an insulating outer layer 25 and comprises a number of conductors 27 for electrically feeding the superconducting coil. Note that FIG. 15B is a cross sectional view taken along line C-Cxe2x80x2 in FIG. 15A.
A conventional superconducting apparatus as described above and illustrated in FIG. 16 has a space 6 in each area of the cryogenic chamber 2 provided for securing a lead to the cryogenic chamber and the space is filled with helium gas. As the superconducting coil is fed with power coming from an external power source via the leads, a high electric field is produced near each of the fitting flanges 5 particularly when an alternating current is used. Since helium gas is a relative poor insulator if compared with air, electric discharges can take place between each of the leads and the corresponding fitting flange through the space 6 surrounding the lead.
The lead and the fitting flange have to be separated from each other by a large distance for electric insulation. This by turn requires a large cryogenic chamber if the superconducting apparatus is to operate stably, making it difficult to down-size the superconducting apparatus comprising such a cryogenic chamber.
Additionally, while each of the leads of a superconducting apparatus comprising a number of conductors 27 is peripherally covered by an insulating layer 25 as shown in FIGS. 15A and 15B, it will be removed from the apparatus whenever necessary and put back to the apparatus again. In view of the convenience with which the operation of removing a lead from and putting it back to the apparatus is carried out, the insulating layer 25 cannot be made too thick and therefore the space 6 cannot be reduced significantly by increasing the thickness of the insulating layer 25 of the lead.
Japanese Patent Application Laid-Open No. 04-320305 proposes an improved superconducting apparatus provided with reduced spaces in areas for securing leads to a cryogenic chamber. The proposed apparatus shows a raised withstand voltage at each area for securing a lead to the cryogenic chamber as a highly insulating gas such as dry air or nitrogen is made to flow through the space surrounding the lead. However, such an apparatus requires an additional device for supplying gas and a specific arrangement has to be made to remove moisture from the gas. Thus, while the proposed apparatus has reduced spaces in areas for securing leads to a cryogenic chamber, it is additionally provided with a device for keeping a gas flow inside the cryogenic chamber and hence it is not significantly down-sized.
The leads of a superconducting apparatus of the type under consideration are required to prohibit intrusion of external heat in addition to its role of feeding power from an external power source. However, since leads are normally made of a metal that is thermally as well as electrically conductive, they can transmit not only electric power but also external heat into the superconducting equipments. A proposed solution to this problem is the use for leads of an oxide superconductor having a low thermal conductivity and no electric resistance. For instance, Japanese Patent Application Laid-Open No. 03-283678 discloses a lead realized by fitting oxide superconductors to a mechanically strong metal core and Japanese Patent Application Laid-Open No. 04-369875 teaches the connection of the oxide superconductor to provide the conductors of a lead. Leads of an superconductor oxide are effective for preventing external heat from entering the cryogenic chamber of a superconducting apparatus because the oxide has a very low thermal conductivity.
However, oxide superconductors are accompanied by a problem that the rate of electric current running therethrough can be remarkably reduced if subjected to an external magnetic field (FIG. 17). Note that a current density as high as about 10,000 A/cm2 is normally required for a superconducting magnet. Thus, while leads realized by using oxide superconductors as a conductor can effectively prevent external heat from entering the cryogenic chamber of a superconducting apparatus, the rate of electric current at which power is fed to the superconducting equipment housed in the cryogenic chamber fluctuates to such an extent that the superconductivity of the equipment becomes totally impaired if an external magnetic field is applied thereto. Particularly if the superconducting equipment comprises a superconducting magnet, the effect of a leaky magnetic field that can be produced by the magnet will constitute a serious problem.
In order for a superconducting apparatus as illustrated in FIG. 16 to operate properly, it is important that not only the superconducting equipment and other components of the apparatus are reliable but also sufficient measures are taken to protect them against electric discharges and electromagnetic noises that can be generated in and near the areas where leads are secured thereto when the superconducting equipment is operated. Existing conventional superconducting apparatuses are, however, not sufficiently protected against discharges and noises and may require additional measures for protection. Furthermore, where a superconducting apparatus comprises a superconducting magnet that requires power to be supplied at an enhanced rate, any trouble on the part of the leads of the apparatus results in a non-operational condition of the superconducting equipment. While abnormal conditions on the part of the leads of a conventional apparatus are detected by measuring the voltage at the opposite ends of each conductor particularly when they are carrying an electric current, such a technique is of no use for protection against electric discharges that may occur between the leads and the cryogenic chamber.
It is therefore the object of the present invention to provide a down-sized superconducting apparatus provided with means for effectively preventing electric discharges in areas for securing leads to the apparaus without using any additional devices and preferably also provided with means for preventing noises so that the leads may operate in a stable manner.
According to the invention, the above object is achieved by providing a superconducting apparatus comprising a cryogenic chamber, a superconducting equipment contained in the cryogenic chamber and a lead secured to the cryogenic chamber and connected to the superconducting equipment, characterized in that a structured member for a prevention of an electric discharge is provided between the lead and an area for securing the lead of the cryogenic chamber.
Preferably, the structured member for the prevention of electric discharge may be a laminate of a conducting layer and an insulating layer.
Preferably, the structured member for the prevention of electric discharge may have an effect of shielding the corresponding lead from electromagnetic noises.
If the structured member for the prevention of electric discharge is a laminate of a conducting layer and an insulating layer, the conducting layer may preferably contain a magnetic substance.